Long-term Objective: Our long-term objective is to elucidate biochemical reactions in heart muscle cells that protect them from damaging stresses, such as deprivation of oxygen and nutrients. The eventual aim of this knowledge is that activation of such reactions could provide a method by which newly developed drugs could protect the heart against the irreversible damage resulting from coronary artery disease. Background: Coronary artery disease reduces blood-flow, which starves the heart of oxygen and nutrients;this creates a condition known as ischemia. Chronic ischemia, or even short-term ischemia followed by resumed blood-flow, which is called ischemia/reperfusion (I/R), can irreversibly damage the heart muscle, causing a myocardial infarction (MI). However, sub-lethal ischemia activates potentially protective biochemical pathways in the heart, one of which may be the unfolded protein response (UPR), also known as the endoplasmic reticulum (ER) stress response (ERSR). The UPR has not been studied extensively in cardiac myocytes, but in other cell types it is activated when protein folding in the rough ER, which requires oxygen and nutrients, is impeded. We found that simulated ischemia (sI) impedes ER protein folding and activates the UPR in cultured cardiac myocytes and in mouse hearts subjected to MI, in vivo. One branch of the UPR is mediated by the transcription factor, ATF6. In other cells types, the UPR activates ATF6, which induces ERSR genes that encode protective proteins. We found that transgenic expression of activated ATF6 induced numerous ERSR genes and proteins, which protected mouse hearts in an ex vivo model of I/R damage. Hypothesis: Our hypothesis is that a group of the genes activated in myocardial cells during ischemia are induced by the ATF6 branch of the UPR, and that these ATF6-dependent genes encode proteins that help protect the heart from I/R damage via appropriate regulation of stress signaling pathways in cardiomyocytes. Research Design: This hypothesis will be addressed in the mouse heart, where ischemia-inducible, ATF6- dependent genes will be identified by gene expression profiling. The mechanisms by which these genes contribute to protection will be examined in cultured cardiac myocytes, and in vivo. The effects of ATF6 loss- or gain-of-function on cardiac performance and damage upon I/R will be examined in the mouse heart, in vivo. Specific Aims: Our specific aims are to: 1. identify ischemia-inducible, ATF6-dependent genes in the mouse heart, in vivo, using unbiased microarray-based gene expression profiling and ERSR-specific gene profiling strategies, 2. determine the consequences of ATF6 gain- or loss-of-function in a cultured cardiac myocyte model of simulated ischemia and I/R, and in a mouse heart model of in vivo I/R, and 3. examine the mechanisms of ischemia-inducible, ATF6-dependent cardioprotection.